The use of ultraviolet (UV) radiation for medical phototherapy is well established. In fact, UV therapy is now involved in the treatment of more than 40 types of skin diseases and disorders such as psoriasis, vitiligo and eczema. Phototherapy studies of UVB wavelengths between 260 nm and 320 nm have found that a narrow-band UVB emission centered at approximately 312 nm is most effective for phototherapy while at the same time limiting undesirable erythemal effects. Since the skin's erythemal (or sunburning sensitivity) is at its maximum at about 297 nm, a narrow-band emission at about 312 nm allows a patient to have longer treatment times before an erythemal response appears.
The Gd3+6P7/2-->8S transitions are ideal for 312 nm narrow-band emissions. However, f-f transitions of rare earths, being parity forbidden, are very weak and the use of a sensitizer is necessary to obtain a useful emission intensity. One of the first narrow-band UVB phosphors to be developed was sensitized with bismuth, e.g., (Gd0.5,La0.487)B3O6:Bi0.013. On excitation by 254 nm radiation, this borate phosphor emits the characteristic radiation with a very narrow band centered on 312 nm. However, because of the toxicity of the bismuth sensitizer, other narrow-band UVB phosphors were developed, in particular YMgB5O10:Gd,Ce (U.S. Pat. Nos. 4,319,161 and 6,007,741), and YMgB5O10:Gd,Ce,Pr (U.S. patent application Ser. No. 10/907,349, filed Mar. 30, 2005).
For the most part, UV-emitting phosphors have been optimized for excitation by the 254 nm emission of the low-pressure mercury discharge used in conventional fluorescent lighting. However, because of environmental concerns, there is a growing need for mercury-free lighting technologies. One such technology is the xenon discharge lamp which produces radiation at about 172 nm in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum. It would be advantageous to develop phosphors which are optimized for excitation in the VUV region and could be used in a Xe-discharge lamp for medical phototherapy.